1. Field of Invention
The present invention relates to an Al—Sn based aluminum-alloy for sliding bearing. More particularly, the present invention relates to an Al—Sn based aluminum-alloy exhibiting improved fatigue resistance at high-temperature region due to dispersion of fine particles, while maintaining the compatibility at a room-temperature region, when used as a sliding bearing. The present invention also relates to a production method of the Al—Sn based sliding bearing, in which fine particles are dispersed.
2. Description of Related Art
Aluminum alloy and copper alloy are two major materials of the sliding-bearing materials. Representative additive components of the aluminum alloy are Sn, Pb and the like, which impart the lubricating property and compatibility, as well as Si and the like which impart the wear resistance.
One means for enhancing the fatigue resistance of the aluminum alloy is to add such elements as Si, Cr, Cu and Mg in some extent as to utilize the precipitation hardening of these elements. The heat treatment for precipitation hardening is usually the solution heat-treatment followed by aging at room temperature (T4) or artificial aging at approximately 150° C. (T6) Another means for enhancing the fatigue resistance is to add such elements as Cu and Mg within the solubility limit and hence to utilize the solution strengthening. The heat treatment usually employed is the solution heat-treatment followed by aging at room temperature. (T4)
The effects of solution strengthening method mentioned above are lost at elevated temperature. Both strength and hardness increase with the temperature increase from room temperature to somewhat high temperature in each case of solution strengthening and precipitation hardening. However, the compatibility, which is important for sliding bearing, deteriorates as strength and handness increase. Along with deterioration of compatibility, there arises danger of seizure and fatigue.
Various proposals have been made to improve the compositions of aluminum alloys described above. A proposal made by one of the present applicants and employed in actual machines is disclosed in German Patent DE 32 49 133 C2. The aluminum-alloy used for sliding bearing proposed in this patent is characterized in that hard particles of Si, Fe and the like having average particle diameter of from 4 to 5 μm are coarsely precipitated. The nodular cast iron of the opposed shaft is shaved by the coarse hard particles, thereby forming compatible bearing-surface and enhancing the bearing performance.
Similar proposal has been made by one of the present applicants and is disclosed in U.S. Pat. No. 4,153,756. The Al—Sn based sliding bearing proposed in the patent contains a small amount of Cr, and prevents the coarsening of the Sn particles due to the effects of Cr and hence the fatigue from occurring.
Meanwhile, it is known to apply the ceramic-particle dispersion strengthening to the aluminum-alloy (for example, Japanese Patent No. 2709097). The aluminum alloy, which is strengthened by the ceramic fine particles, is usually produced by the powder metallurgy method. This alloy is appropriate for the wear resistant parts. But can not meet sever compatibility which may be required for the sliding bearing.
It is also known to add the ceramic particles to molten aluminum-alloy. For example, the ceramic particles are added during the die casting (Japanese Patent No. 2739580). In Japanese Unexamined Patent Publication No. 6-17165, the ceramic particles fed into the melt from mother alloy. A green compact consisting of Ti powder, graphite powder and Al (alloy) powder is prepared and is then impregnated with the Al (alloy) melt, followed by heating to form TiC particles. The so treated green compact is used as the mother alloy of TiC.
When the ordinary aluminum alloy is compared with the composite ceramic-aluminum alloy, hardness at room temperature and compatibility of the former are lower and higher, respectively, than these of the latter. However, the hardness of the former abruptly drops at high temperature so that the fatigue resistance becomes unsatisfactory. On the other hand, since the latter is harder at high temperature than the former, the fatigue resistance of the latter is superior to that of the former. The compatibility of latter is poor due to high hardness at room temperature.